1. Field of the Invention
The present invention relates generally to detecting RF field strength, and, in particular, to detecting RF field strength in a passive RFID system.
2. Description of the Related Art
In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art that should be familiar to those skilled in the art of radio frequency (“RF”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well know, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
In accordance with our prior invention previously disclosed in the Related References, the amplitude modulated (“AM”) signal broadcast by the reader in an RFID system will be electromagnetically coupled to a conventional antenna, and a portion of the current induced in a tank circuit is extracted by a regulator to provide operating power for all other circuits. Once sufficient stable power is available, the regulator will produce, e.g., a power-on-reset signal to initiate system operation. Thereafter, the method disclosed in the Related References, and the associated apparatus, dynamically varies the capacitance of a variable capacitor component of the tank circuit so as to dynamically shift the fR of the tank circuit to better match the fC of the received RF signal, thus obtaining maximum power transfer in the system.
In general, the invention disclosed in the Related References focused primarily on quantizing the voltage developed by the tank circuit as the primary means of matching the fR of the tank circuit to the transmission frequency, fC, of the received signal. However, this voltage quantization is, at best, indirectly related to received signal field strength. We submit that what is needed now is an effective and efficient method and apparatus for quantizing the received field strength as a function of induced current. It is further desirable to develop this field quantization in a form and manner that is suitable for selectively varying the input impedance of the receiver circuit to maximize received power, especially during normal system operation. Additionally, in light of the power sensitive nature of RFID systems, it is desirable to vary the input impedance with a minimum power loss.